Gun system comprising an ion trap



NOV. 4, 1958 w, BERTHOLD 2,859,364

GUN SYSTEM COMPRISING AN ION TRAP Filed May 4, 1956 INVENTOR W- B E RT H O L D 'B'Y I ATTORNEY Patented Nov. 4, 1958 2,859,364 I v GUN SYSTEM COMPRISINGAN ION Wolfgang lierthold, Stnttgart-Weil' Dorf, Germany,

assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 4, 1956, Serial No. 582,719 Claims priority, application Germany May 26, 1955 4 Claims. (Cl. 313-75) The invention relates to a beam generating system comprising an ion trap, and in particular with such an ion trap in which the electron beam is redirected to the beam axis by means of a magnetic field produced by a permanent magnet arranged within the tube.

The employment of ion traps is particularly appropriate with such tubes in which the electron beam is scanned over the fluorescent screen by means of magnetic deflecting fields, because the negative ions are not deflected by the magnetic fields. These negative ions, therefore, always impinge the fluorescent screen at the same spot, thus damaging the fluorescent substance in the course of time, so that gradually there will be noticed a dark spot mostly in the center of the picture screen on the fluorescent screen.

As a rule the ion trap consists of an electric transverse field and of a magnetic compensating field for the electrons, the latter serving to re-direct the electrons to the beam axis, so that they hit the center of the fluorescent screen, but not affecting the negative ions, so that they may be trapped on a diaphragm. In most cases the magnetic field is produced by a permanent magnet arranged outside the tube. In order that the electrons impinge on the desired spot of the fluorescent screen, this permanent magnet has to be adjusted correspondingly, which often renders difliculties, because the magnet, which only can be adjusted properly by skilled persons, is easily displaced during transporting, which has a deteriorating efiect upon the picture brightness and sharpness.

It has already been suggested to build-up the ion trap magnet as a permanent magnet, previously adjusted, into the picture or image tube and to adjust the magnetic field strength to a desired anode voltage. Thus, for example, arrangements are known in which the magnet is placed in the anode cylinder. Such arrangements, however, have proved to be unsuitable, because the field of the magnet, due to the relatively small dimensions, becomes very inhomogeneous, so that the electron beam will be substantially distorted. Furthermore, arrangements have been proposed in which the field of a small permanent magnet, which is arranged at the side outside the tube axis, is led to the axis within the tube by means of two pole shoes. However, also with such arrangements the magnetic field becomes very inhomogeneous and is the cause of the previously mentioned distortions of the electron beam.

In the case of gun systems comprising ion traps and in which the electrons are redirected to the axis by means of the magnetic field of a permanent magnet arranged inside the tube, the invention proposes to arrange the permanent magnet assembly, which is necessary for producing the magnetic field, symmetrically in relation to the axis. It is of a particular advantage to arrange the magnet assembly or the point of maximum field strength a in the system axis behind the cathode plane. In this way only the one side of the magnetic field is being utilised through which the electrons pass and it will further be achieved that the field becomes relatively short and very homogeneous, thereby avoiding the distortion of the electron beam. .The short magnetic field particularly favorable because thereby, for, example, when. employing an inclined or tapered electron lensfor pro ducing the transverse field, or when using an asym=' metrical diaphragm at one of the accelerating electrodes, a chamfering or tapering of the beam generating system will be superfluous.

The invention will be particularly described hereinafter with reference to the exemplified embodiments shown in the accompanying drawings in which:

Figs. 1 and 1A diagrammatically illustrate a longitudinal sectional view and an end view respectively of a gun according to the invention.

Fig. 2 diagrammatically illustrates a longitudinal section of a gun showing a modified form of the invention.

In Fig. 1 is shown a gun system comprising an ion trap, consisting of the cathode 1, of the control electrode 2, and of the accelerating electrode 3. Further accelerating electrodes are not particularly designated, because they are of no special importance to the understanding of the invention. Likewise the means for production of the electric transverse field is not shown in detail, because the invention is compatible with all conventional systems for generating the electric transverse field. Filaments and other component parts of the tube, which do not contribute towards a better understanding of the invention, are omitted for reasons of clarity. Around the control electrode 2 there are arranged two bows or clips 4 which, at least partly, may consist of ferromagnetic material, such as nickel 'and iron, and may be firmly connected with the control electrode, for example, welded. These clips serve to hold in position two permanent magnets 5 consisting for example of aluminum-nickel or aluminum-nickel-cobalt or of any other conventional magnet alloy. As particularly advantageous it is regarded by the invention to employ magnet material that has been sintered in vacuum. Appropriately the magnets may be held between the clips by any desired means such as depressions formed therein. The magnets may consist of one single ring which is magnetised perpendicularly in relation to its symmetry axis if desired instead of the separate units as shown. Further, it is possible to attach the magnet arrangement to another electrode, for example to the screen grid.

An even better arrangement is shown in the embodiment of Fig. 2, in which the magnet assembly 5 is formed by a permanent magnet. positioned entirelybehind the cathode. In order to arrange the magnet 5 as close as possible behind the surface of the cathode 1, this cathode is arranged parallel in relation to the control electrode 2. While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof an in the accompanying claims.

What is claimed is: l. A gun system including a cathode and electron beam forming electrodes, an ion trap in which the electrons of an electron beam are redirected to the tube axis by means of a magnetic field produced by a perma- 3 3. A gun system as in claim 1, having a control electrode surrounding said cathode, characterised in that said magnet assembly positioned outside said control elec- "4E A gmi system as ifi'clai m'fi, *chara'cteris'e'd'by 'ele' 5 lfie'nts of ma netic material ifigrcennectingisaid' magne'ts.

I References Cited in the file of this patent UNITED STATES PATENTS Ploke Feb. 28, 1939 Nicoll Nov. 28, 1939' Holst Aug. 20, 1940 Gethmann June 15, 1954' 

